In vertebroplasty, the surgeon seeks to treat a compression fracture of a vertebra by injecting bone cement such as PMMA into the fracture. In one clinical report, Jensen et al., AJNR: 18 Nov. 1997, Jensen describes mixing two PMMA precursor components (one powder and one liquid) in a dish to produce a viscous bone cement; filling 10 ml syringes with this cement, injecting it into smaller 1 ml syringes, and finally delivering the mixture into the desired area of the vertebral body through needles attached to the smaller syringes.
Although this procedure has found success, it has also met with some problems. For example, Jensen reports that the open-air mixing of the precursor materials produces problematic fumes. Also, when the plastic plunger in a 1 mm syringe is pressed against the viscous cement in the syringe barrel, the appreciable resistance of the cement to this pressure may cause the relatively thin plunger rod to collapse. To minimize this danger, only about half of the syringe barrel is typically filled with cement. Although this precaution enhances the feasibility of the procedure, it requires loading twice as many syringes, thereby increasing the total time of the procedure.
In addition, the pressures which can be generated by manual operation of 1 ml syringes are fairly low. Referring now to FIG. 7, the pressure generated in a fluid within a conventional syringe S is governed by the equation P=F/A, wherein:                P=the pressure at the cement delivery opening O,        A=the cross-sectional area of the disc D, and        F=the force applied to the handle H in a direction normal to disc D.        
Since about 10–15 pounds of force (˜44–66 N) is typically generated by manually pressing handle H, and the disc area A of a typical 1 ml syringe is about 0.025 in2 (˜16.129 mm2), the pressure P available for injecting the bone cement is only about 400–600 psi (˜2.75–4.13 MPa). In vertebroplasty performed with these methods, a pressure of at least about 1000 psi (˜6.9 MPa) is desirable, with a pressure of about 2000 psi (˜13.78 MPa) being more preferable.
WO 99/49819 (“Preismann”) recognized the problem of low delivery pressures in conventional syringes and suggested using a screw-type plunger in a single injection chamber. Although Preismann's screw-type plunger provides more pressure per unit force than the conventional plunger of FIG. 7, it nonetheless requires a cross-sectional area A of about 0.5 in.2 (322 mm2) in order to deliver 2000 psi (˜13.78 MPa), and so a chamber capable of delivering 20 cc of bone cement would need to be about 6.22 inches (158 mm) in length. Since this device would likewise require a screw-type plunger of equal length, the total length of the device with the plunger extend would be at least about 12.44 inches (63.5 mm), which is fairly long for a surgical setting. One apparent commercial embodiment of Preismann uses an injection chamber having a length of about 2.5 inches and holds only about 8 cc of bone cement. In addition, Preismann only discloses a delivery system. The user must still mix the two bone cement precursors outside the delivery system, and then transfer the mixed cement to the delivery system.
U.S. Pat. No. 5,252,301(“Nilson”) discloses a bone cement delivery apparatus wherein the two bone cement precursors are mixed within the injection chamber, thereby solving the fume and transfer problem. However, Nilson discloses a conventional delivery system having a conventional plunger in a single chamber for delivering the bone cement, and so does not provide high pressure delivery.
There are a number of gun-type injectors which seek to use the mechanical advantage of a lever to deliver high pressure fluids from a chamber into the patient. See, for example, U.S. Pat. No. 4,338,925. However, in order to provide pressures of about 2000 psi via a conventional 11.4 cm2 (1.77 inches2) disc in a conventional single chamber device used in orthopaedic applications, the mechanical advantage would have to be very high.
U.S. Pat. No. 5,190,191 (“Reyman”) discloses a dual chamber delivery device intended to deliver contraceptive fluid, wherein the fluid is first transferred from a large reservoir into a smaller delivery chamber and then delivered from the delivery chamber by means of a conventional syringe-type plunger. As fluid is delivered via pressure developed by a conventional plunger, this device suffers from the same low pressure deliveries experienced by syringes using conventional plungers. In addition, the ratio of the cross-sectional area of the reservoir-disc to the injection-chamber disc appears to be less than about 8:1. As Reyman addressed the problem of delivering known, discrete quantities of fluid and was not concerned with providing high pressure fluids, the low pressure delivery characteristics of the Reyman device was not a particular concern to Reyman. In addition, Reyman's preferred embodiment uses a one-way valve between the reservoir and dispensing chamber (to prevent back flow to the reservoir during delivery) and a cap upon the opening in the dispensing chamber (to prevent loss of the fluid during inter-chamber transfer). These features make this device very complicated to operate. Lastly, Reyman does not disclose an injection chamber having a sterile inner surface.
There are a number of pneumatic injection devices disclosed in the literature as well. However, these devices often provide inadequate control of the delivery of the bone cement, and may pose safety concerns in applications such as vertebroplasty wherein control of the injected material is of paramount importance.
U.S. Pat. No. 6,033,105 (“Barker”) discloses an integrated bone cement mixing and delivery system. However, Barker teaches using an auger mechanism as the means for advancing the bone cement through the delivery chamber. Barker teaches that the auger mechanism is advantageous because it provides both controlled delivery and high pressure. Barker further discloses many alternative embodiments of the device, wherein the pitch, direction and diameter of the augur threads are varied in order to maximize control, enhance delivery pressure and eliminate air bubbles in the bone cement.
The English language abstract of JP 10146559 discloses a device for injecting viscous fluids in discrete quantities which requires a reciprocating holding member (3). The English language abstract of JP 10146559 does not disclose an injection chamber having a sterile inner surface, nor does fluid communication ever exist between the reservoir and injection chamber exit openings.
U.S. Pat. No. 5,931,347 (“Haubrich”) discloses a device for injecting viscous fluids in which the injection chamber has an axially movable piston which a) when moved from the entry opening of the injection chamber towards the chamber's exit opening, creates a vacuum in the chamber in order to draw fluid from reservoir into the chamber, and b) when moved in the opposite direction, has a through hole to allow the fluid to move through it to reach the exit opening of the injection chamber. Haubrich does not disclose an impermeable piston in the injection chamber.